Heretofore, machines for fabricating by resistance welding screens, mats or like structures formed of crossed, laterally spaced longitudinal and transverse wires, rods or bars, have been provided for each crossing point at the welding station at which the welding operation is performed, either with a pair of upper and lower electrodes, the lower of which usually is stationary and the upper vertically movable, as in Gott U.S. Pat. No. 3,588,417, or, if a pair of spaced transverse wires are simultaneously welded to the longitudinal wires, with a pair of vertically movable upper electrodes for the corresponding crossing points of the transverse wires, with welding current flowing between the electrodes through a conductive anvil underlying both points, as in Smith et al U.S. Pat. No. 3,961,153.
An alternative, provided in Fotie U.S. Pat. No. 2,422,829, is a vertically movable upper electrode for each crossing point of a row extending across the longitudinal wires and a common lower electrode of like extent. If, as in Fotie, the current is supplied to the electrodes through transformers, the transformers usually are stationary and connected to the electrodes by flexible secondary leads, with consequent reduction, from the increased length of the leads, in the current available at the electrodes. This loss, Gott et al in U.S. Pat. No. 4,149,059, sought to minimize by mounting transformers for adjoining rows of vertically movable upper electrodes in a common pressure bar, with the secondary leads of the transformers for each row connected by sliding contacts to the electrodes of that row.
As opposed to conventional resistance welding machines in which the upper electrodes either are movable only vertically or, as in Gott et al U.S. Pat. No. 3,588,417, are also adapted for limited transverse movement to permit the spacing of the longitudinal wires to be varied, by providing for each pair of electrodes a pair of contact shoes slidable on conductor rails, Osborn in U.S. Pat. No. 1,042,466, discloses a so-called "traveling electric welding apparatus" in which a carriage riding on flanges of an overlying beam, has suspended therefrom a transformer, the flexible secondary leads of which in turn suspend a pair of electrodes. A threaded connection between the transformer and the carriage, through which the transformer and electrodes are movable vertically in unison, enables the electrodes to be adjusted vertically to suit the particular workpiece in the welding station, but only by manual actuation of the pivoted jaws mounting them can the electrodes exert pressure on the unnamed workpiece grasped between them. The earlier mentioned Smith et al patent also discloses the mounting of its pair of electrodes in a welding head in turn mounted in a carriage drivable intermittently along a welding station for enabling the pair of electrodes to sequentially weld a pair of transverse wires at all crossing points to the longitudinal wires but mentions neither a transformer nor any relation of one to either the carriage or the welding head.
The present invention is particularly concerned with an improved resistance welding machine, in which a welding unit is movable transversely of longitudinal wires along a welding station for welding a transverse wire to the longitudinal wires.